The abundances of ClO have been measured, in situ, in the lower stratosphere during sunrise and sunset. Measurements were made with an instrument mounted on the NASA ER-2 aircraft, which was flown at an altitude of 20 km and latitudes between 35¿N and 47¿N during a morning and an evening flight. The abundances of ClO were observed over a dynamic range of 20 from a detection threshold of ~1 part per trillion volume (pptv). These data confirm the sunrise variation of the photolysis of cholorine nitrate that is predicted by a zero-dimensional photochemical model. They also suggest that the absolute photolysis and termolecular formation of chlorine nitrate occur at rates consistent with nominal ClONO2 concentrations.